Fire Forming with Factory Ammo

As long as you don't have a tight neck chamber in the 243AI, you're usually good to go with shooting factory 243 ammo or handloaded ammo that's been FL sized (within reason). If you feel a slight resistance when chambering a round, that is good.

The AI chamber should be just a tad shorter at the neck/shoulder junction in a perfect world. That's how they were designed to be anyway. Just as long as it's not longer than factory headspace.........that's when we need to seat bullets out long or create a false shoulder on the neck.

A rifle that's properly set up should shoot well under moa while fireforming. I've even heard tell that some have actually gotten better groups while fireforming.? When I was forming some 6 long dasher stuff a few months ago, I was getting some 1/2 moa groups at 300 and 400 yds just shooting off the hood of the pickup.

Not freehand, LOL. I wish I could shoot that good with only my hands.
I had a sandbag under the forend and a rolled up coat under the butt. But I was standing up and resting on the hood of the truck, no bench no bipod.

My "new to me" 243AI seems to shoot pretty well forming brass too. No problems hitting prarie dogs at 350 yds with cheap factory ammo........as long as I called the wind right.